But before then, the Duchess, who turns 37 next month, is expected to travel solo to California and New York for a "personal trip", a palace source revealed.

"She's planning a personal, unofficial trip this summer to see family and friends," the source told Us Weekly.

"She'll go to New York, then LA."

Prince Harry and Meghan, the Duchess of Sussex, look at a Shakespeare book favoured by prisoners on Robben Island with Andrew Miangeni, who was in prison with Nelson Mandela, at the launch of the Nelson Mandela Centenary Exhibition, marking the 100th anniversary of anti-apartheid leader's birth.Arthur Edwards/AAP

The royal couple - who tied the knot in May - have already announced a trip to Sydney in October for Harry's Invictus Games as part of their first overseas tour as a married couple.

The itinerary also includes stops in Fiji, Tonga and New Zealand.

While Harry and Meghan's US tour is still being finalised, the royal couple want it "to solidify ties between the UK and the US", a source said.

"They want to highlight and foster these relationships."

In San Francisco, the couple "will meet with CEOs and female tech entrepreneurs to encourage their involvement in the industry".

A possible visit with former President Barack Obama, and his wife Michelle Obama, the former First Lady, is also on the cards.

Australia is one of the first countries the royals will visit since their wedding.

Prince Harry and the former Suits star will attend the Invictus Games in Sydney, which is on from October 20 to 27.

Harry founded the Invictus Games in 2014 for wounded, injured and ill veterans and serving defence personnel.

More than 500 athletes from 18 countries will compete in the Sydney event, to be held at Olympic Park.

The couple made their first official public appearance at the Invictus Games in Toronto last year.